Plasmas are used for many processes to alter the surface of a material, such as an integrated circuit chip wafer, in a controlled manner. Typical applications include the etching of semiconductor wafers, ion implantation, ion milling, and the removal of material in a process known as reactive ion etching. The fabrication of an integrated circuit usually begins with a thin, polished slice of a high purity, single-crystal semiconductor material, such as silicon, or germanium, which is subjected to a sequence of processing steps, such as deposition of materials on, or removing materials from the wafer to form the structured layers of the integrated circuit. Early etching techniques were based on chemical etching agents. Early deposition processes included sputtering or chemical vapor deposition techniques. More recently, etching and deposition processes based on energetic plasma ions taking place in a plasma reactor, have been replacing the earlier techniques because of environmental and health concerns, in addition to the improved quality which results from the plasma process.
Plasma reactors typically include a chamber in which the plasma is established, a source of gas which is ionized to form the plasma, a source of energy to ionize the gas, a vacuum system to reduce the pressure within the chamber to the appropriate level for the particular process, and a means for allowing the gas to enter the chamber in a controlled manner.
Generally, the item to be processed, which may typically be a semiconductor wafer with appropriate masking, is placed within the chamber and is electrically biased relative to the gas in order to induce the charged species of the ionized gas to impinge on the wafer preferably substantially normal to the surface. In some cases, chemically reactive gases are also used to enhance the rate of etching in a process called reactive ion etching.
In a typical plasma etching system, the chamber has an upper electrode which serves as an anode, and a lower electrode which serves as a cathode. The item to be processed is generally mounted on the cathode. In such a system, the cathode is biased negatively with respect to the anode and the chamber walls, and thereby establishes an electric field between the cathode and the surrounding region. The electric field gradient established by the negatively biased cathode is sufficiently strong enough to dissociate the particular gas used in the chamber, at the operating pressure to form a plasma. The dissociated gas results in charged particles in the form of electrons and positively charged ions, each of which are accelerated by the electric field. The surface of the item to be processed is etched by the positive ions that are accelerated towards the negatively charged cathode by the electric field. It is generally important in semiconductor processing that the ions strike the wafer with a uniform flux density so that all parts of the wafer be processed at the same rate, and that the ions follow a path that is perpendicular to the surface of the wafer in order to prevent defects associated with undesirable etching of the sides of the semiconductor structures. It is also important that extraneous particles which can cause defects be eliminated, and that the components of the reactor be easily replaced or serviced.
Generally the upper electrode, the anode, is provided with gas holes in a distribution that encourages an even distribution, and uniform flow, of plasma over the surface of the article to be processed. Typically, the anode is comprised of a silicon disk of uniform thickness, and is referred to as a showerhead electrode in the art. When gases which are chemically reactive with the material to be etched, are used in the process known as reactive ion etching, the energetic etching properties of the plasma process are combined with the chemical etching effect of the reactive gas. Reactive gases typically contain compounds of highly reactive elements, such as chlorine, bromine or fluorine, generally in a mixture with other gases such as oxygen, neon, argon, or sulphur. Other compounds, using other chemicals are also well known in the art.
One consequence of using reactive gases is that the reactive gases cause excessive electrode wear. Also, the electrons which are created by the dissociated gas are accelerated towards the anode, and cause etching and wear. A detrimental effect of the anode wear, is that the holes which form the gas inlet may change over time, and effect the process.
In certain known showerhead configurations, the silicon gas plate is secured to the graphite ring using Indium which is known to bond silicon to graphite. However, there are a number of disadvantages with using Indium in the plasma reactor. There is evidence that Indium has a low level of toxicity and thus has handling and disposal problems. Indium is relatively expensive. Indium also has a high vapor pressure, which means that it easily outgases, which in the plasma reactor chamber vacuum, causes outgassing of particulates, which contributes to a creating a dirty chamber. The process for soldering the silicon gas plate to the graphite ring with Indium is difficult, because a flux cannot be used. The assembly process requires heating the silicon and graphite to drive off impurities, and then rubbing the silicon and graphite with pure Indium to produce oxides. The Indium solder then bonds to the Oxides, which is bonded to the silicon and graphite parts. The resulting solder bond is in mechanical stress, and being a relatively soft material, tends to flow since it is in a state of shear. Another disadvantage with the use of an Indium bond, is that it is the only structural support holding the silicon gas plate to the graphite ring, and if the temperature of the gas plate and ring approaches 150 C., such as may happen if the plasma process runs hot, the gas plate may fall, damaging the wafer or the reactor. Finally, the Indium solder bond is thermally cycled during the manufacturing process, which could weaken the oxide bonds, also causing a failure.
Consequently, it is desirable to have an anode showerhead assembly that is structurally reliable, and is easy to assemble and disassemble. Also, it is desirable to have a showerhead assembly which is made of materials which are compatible with the plasma and reactive etching processes in the reactor. It is also desirable to have a showerhead assembly in which the hole pattern can be changed to accommodate the process, when gas mixture, flow rate, wafer size and pressure is changed to accommodate each particular process.